The present invention relates to a color electrophotographic copying process for producing colored copies.
Color electrophotographic copying systems utilizing the Carlson process are well known in the art. The typical duplicating process in such color electrophotographic copying systems is described below.
A uniformly charged photosensitive body having photoconductivity is exposed to a light image of a colored original through a red filter. The exposure of a photosensitive body to a colored original through a filter will be referred to as "color-separated exposure". An electrostatic latent image formed as a result of color-separated exposure will be referred to as a "color-separated latent image". The color-separated latent image formed by the color-separated exposure is then developed by a toner (hereinafter called a "cyan toner") colored in cyan which is complementary to the color of the filter, i.e., red, that was used in achieving the color-separated exposure. The developed visible image in cyan is then transferred to a white recording sheet as of paper.
The photosensitive body is thereafter subject to color-separated exposure using a green filter, and the resultant color-separated latent image is developed by a magenta toner colored in magenta which is complementary to green. The developed visible image in magenta is also transferred to the recording sheet.
Then, the photosensitive body is subject to color-separated exposure using a blue filter, and the resultant color-separated latent image is developed by a yellow toner. The developed visible image in yellow is also transferred to the recording sheet.
Consequently, the visible imates in cyan, magenta, and yellow are successively transferred to the recording sheet in an overlapping manner to thereby reproduce a colored image of the colored original.
The term "blue" used in the specification is employed to actually mean a hue called "bluish purple".
The individual color-separated latent images obtained by color-separated exposure will be identified by the colors of the toners with which the latent images are to be developed. For example, the cyan latent image means the color-separated latent image to be developed by the cyan toner, and is formed by the color-separated exposure using the red filter. Likewise, the magenta and yellow latent images are the color-separated latent images are formed by the color-separated exposure using the green and blue filters, respectively.
In the aforesaid color electrophotographic copying system, the color of a colored copy image is dependent on the combination of toner colors on a recording sheet. Therefore, the tone of the colored copy image varies if the relative distribution of the quantities of the color toners which constitute the colored copy image is changed.
To stabilize the tone, it has been customary to effect color balancing in the color electrophotographic copying system. Even if the color balancing is carried out ideally, the color of the colored copy image is not an exact duplication of the color of the colored original copied. The reason for this resides in the colors of the toners used.
Now, take the cyan toner for example. Ideal cyan toner is a toner which completely absorbs red light and completely reflects green light and blue light. Any actually available cyan toners, however, does not fully absorb, but slightly reflects, red light, and absorbs green light and blue light partially. Actual magenta and yellow toners are also different ideal magenta and yellow toners, respectively, in the same manner as described above.
An actual cyan toner which absorbs blue and green light partially can be interpreted as containing magenta and yellow toners as impurities. Therefore, a visible image obtained by developing a cyan latent image with an actual cyan toner contains magenta and yellow toners as impurities. When overlapping a visible image in magenta on a visible image in cyan, any magenta toner contained as an impurity in the visible cyan image may be subtracted in advance from the visible magenta image to improve the color reproducibility of the resultant blue image produced as the mixture of the magenta and cyan images.
Such improvement of the color reproducibility of a colored copy image is called "color compensation".
The color compensation is orginally used in the field of color printing. One known process for actually effecting the color compensation is a masking process. It is expected that the masking process can be incorporated in the color electrophotographic copying system for improving the color reproducibility of colored copy images.
Japanese Laid-Open Patent Publication No. 53(1978)-3232 discloses such a masking process as incorporated in a color electrophotographic copying system. To implement the disclosed process, however, a pair of photosensitive bodies having photoconductivity is required and the resultant copying apparatus is large in size.
As well known in the art, it is difficult for an electrophotographic copying system to accomplish proper gradation reproducibility. This results from the fact that the gradation reproducibility curve is not linear on a graph having an original image density on the horizontal axis and a copied image density on the vertical axis.
Since the gradation cannot accurately be reproduced, the tone of a colored copy image in a color electrophotographic copying system is liable to differ from that of the original colored image.